Chitin and chitin synthase do not exist in man. However, chitinases and chitinase-like proteins (C/CLP) have recently been appreciated. This includes true chitinases and moieties that lack chitinase activity like breast regression protein-39 (BRP-39) and its human homologue YKL-40. BRP-39 and YKL-40 are expressed in an exaggerated fashion in a variety of diseases. However, virtually nothing is known about their roles in mammalian or human biology. We studied the regulation and roles of BRP-39 in allergic inflammation. These studies demonstrate that (a) BRP-39 is prominently induced at sites of Th2 and IL-13-induced inflammation, (b) BRP-39-/- mice have a significant defect in Th2 and IL-13-induced inflammation and remodeling that is associated with enhanced T cell and macrophage apoptosis and Fas expression and (c) BRP-39/YKL-40-induced cytoprotection is associated with enhanced protein kinase B/Akt activation. They also demonstrate that BRP-39/YKL-40 binds to IL-13 receptor (R)a2 and activates mitogen activated protein kinases (MAPK) and PKB/Akt via an IL-13Ra2- dependent mechanism. Lastly, they highlight the human relevance of these findings by demonstrating that YKL-40 is found in exaggerated quantities in the serum and lungs from severe asthmatics and that chitinase 3- like 1 is an asthma susceptibility gene. This led us to the following hypothesis. HYPOTHESIS: 1. BRP-39/YKL-40 is induced during and plays a critical and selective role in the pathogenesis of adaptive Th2 inflammation and remodeling. 2. BRP-39 and YKL-40 are important regulators of T cell and macrophage apoptosis/cell death. 3. BRP-39 and YKL-40 mediate their tissue responses via a pathway(s) that involves IL-13Ra2, MAPK and or PKB/Akt. To test this hypothesis we propose to: AIM 1. Characterize the expression and roles of BRP-39 in Th2 and Th1 inflammation and remodeling. AIM 2. Characterize the relative contributions of serum and tissue and epithelial- and macrophage-derived BRP-39/YKL-40 in Th2 and IL-13-induced inflammation and remodeling. AIM 3. Define the interactions of BRP-39/YKL-40 and IL-13Ra2 and the roles of IL-13Ra2 in the pathogenesis of the biologic effects of BRP-39/YKL-40. AIM 4. Characterize the mechanisms of the exaggerated T cell and macrophage apoptosis/cell death responses in BRP-39-/- mice and the relevance of this cell death pathway to humans.